High IQ Brains and Reaction to Stimuli
High-IQ brains also react more quickly to stimuli. Multiple studies have shown that IQ is positively correlated with multiple measures of nerve conduction velocity. The simplest example: A light flashes in front of your eyes, and a machine measures how long it takes for the message to reach the vision centers in the back of your brain. This speed is correlated (0.4) with a person's IQ.
These kind of results should eliminate claims that IQ is merely a measure of acculturation. Instead, IQ measure something much more like processing speed. It is not a 100 percent accurate measure of processing speed. It is an indirect measure rather than a direct measure. Using an IQ test to measure processing speed is akin to measuring processing speed by using a computer to an hour or two rather than precisely counting millions of instructions per second.
Further, it is entirely unclear whether the cognitive correlates of IQ are mere outgrowths of the underlying processing speed (as faster computers routinely run a variety of software more efficiently) or instead reflect separate mental skills that are routinely bundled together with high processing speed (as faster computers are often bundled with better graphics cards).
-- Garett Jones
Quoted on Tue Aug 2nd, 2011